FILE - In this July 5, 2012 file photo, a dock extends into a dry cove at Morse Reservoir in Noblesville, Ind., as oppressive heat and drought conditions stifle the middle of the United States. In 2012 many of the warnings scientists have made about global warming went from dry studies in scientific journals to real-life experience. One example: July was the hottest month in record-keeping U.S. history. (AP Photo/Michael Conroy, File)

FILE - In this July 5, 2012 file photo, a dock extends into a dry cove at Morse Reservoir in Noblesville, Ind., as oppressive heat and drought conditions stifle the middle of the United States. In 2012 many of the warnings scientists have made about global warming went from dry studies in scientific journals to real-life experience. One example: July was the hottest month in record-keeping U.S. history. (AP Photo/Michael Conroy, File)

FILE - In this Wednesday, Aug. 1, 2012 file photo, cattle walk in a dusty pasture in rural Parker County near Springtown, Texas. Nearly 4 out of 5 Americans now think temperatures are rising and that global warming will be a serious problem for the United States if nothing is done about it, a new Associated Press-GfK poll finds. Belief and worry about climate change are inching up among Americans in general, but concern is growing faster among people who don't often trust scientists on the environment. In follow-up interviews, some of those doubters said they believe their own eyes as they've watched thermometers rise, New York City subway tunnels flood, polar ice melt and Midwestern farm fields dry up. (AP Photo/LM Otero, File)

FILE - In this Wednesday, Aug. 1, 2012 file photo, cattle walk in a dusty pasture in rural Parker County near Springtown, Texas. Nearly 4 out of 5 Americans now think temperatures are rising and that global warming will be a serious problem for the United States if nothing is done about it, a new Associated Press-GfK poll finds. Belief and worry about climate change are inching up among Americans in general, but concern is growing faster among people who don't often trust scientists on the environment. In follow-up interviews, some of those doubters said they believe their own eyes as they've watched thermometers rise, New York City subway tunnels flood, polar ice melt and Midwestern farm fields dry up. (AP Photo/LM Otero, File)

FILE - In this Wednesday, Aug. 8, 2012 file photo, a dry corn field receives some rain from a passing thunderstorm near Blair, Neb. Nearly 4 out of 5 Americans now think temperatures are rising and that global warming will be a serious problem for the United States if nothing is done about it, a new Associated Press-GfK poll finds. Belief and worry about climate change are inching up among Americans in general, but concern is growing faster among people who don't often trust scientists on the environment. In follow-up interviews, some of those doubters said they believe their own eyes as they've watched thermometers rise, New York City subway tunnels flood, polar ice melt and Midwestern farm fields dry up. (AP Photo/Nati Harnik, File)

FILE - In this Wednesday, Aug. 8, 2012 file photo, a dry corn field receives some rain from a passing thunderstorm near Blair, Neb. Nearly 4 out of 5 Americans now think temperatures are rising and that global warming will be a serious problem for the United States if nothing is done about it, a new Associated Press-GfK poll finds. Belief and worry about climate change are inching up among Americans in general, but concern is growing faster among people who don't often trust scientists on the environment. In follow-up interviews, some of those doubters said they believe their own eyes as they've watched thermometers rise, New York City subway tunnels flood, polar ice melt and Midwestern farm fields dry up. (AP Photo/Nati Harnik, File)

FILE - In this Oct. 30 2012 file photo provided by the Metropolitan Transportation Authority, the South Ferry subway station in New York City is filled with seawater and debris from Superstorm Sandy. Nearly 4 out of 5 Americans now think temperatures are rising and that global warming will be a serious problem for the United States if nothing is done about it, a new Associated Press-GfK poll finds. Belief and worry about climate change are inching up among Americans in general, but concern is growing faster among people who don't often trust scientists on the environment. In follow-up interviews, some of those doubters said they believe their own eyes as they've watched thermometers rise, New York City subway tunnels flood, polar ice melt and Midwestern farm fields dry up. (AP Photo/ Metropolitan Transportation Authority, File)

FILE - In this Oct. 30 2012 file photo provided by the Metropolitan Transportation Authority, the South Ferry subway station in New York City is filled with seawater and debris from Superstorm Sandy. Nearly 4 out of 5 Americans now think temperatures are rising and that global warming will be a serious problem for the United States if nothing is done about it, a new Associated Press-GfK poll finds. Belief and worry about climate change are inching up among Americans in general, but concern is growing faster among people who don't often trust scientists on the environment. In follow-up interviews, some of those doubters said they believe their own eyes as they've watched thermometers rise, New York City subway tunnels flood, polar ice melt and Midwestern farm fields dry up. (AP Photo/ Metropolitan Transportation Authority, File)

FILE - In this July 26, 2011 file photo, a Greenlandic Inuit hunter and fisherman steers his boat past a melting iceberg, along a fjord leading away from the edge of the Greenland ice sheet, near Nuuk, Greenland. Nearly 4 out of 5 Americans now think temperatures are rising and that global warming will be a serious problem for the United States if nothing is done about it, a new Associated Press-GfK poll finds. Belief and worry about climate change are inching up among Americans in general, but concern is growing faster among people who don't often trust scientists on the environment. In follow-up interviews, some of those doubters said they believe their own eyes as they've watched thermometers rise, New York City subway tunnels flood, polar ice melt and Midwestern farm fields dry up. (AP Photo/Brennan Linsley, File)

FILE - In this July 26, 2011 file photo, a Greenlandic Inuit hunter and fisherman steers his boat past a melting iceberg, along a fjord leading away from the edge of the Greenland ice sheet, near Nuuk, Greenland. Nearly 4 out of 5 Americans now think temperatures are rising and that global warming will be a serious problem for the United States if nothing is done about it, a new Associated Press-GfK poll finds. Belief and worry about climate change are inching up among Americans in general, but concern is growing faster among people who don't often trust scientists on the environment. In follow-up interviews, some of those doubters said they believe their own eyes as they've watched thermometers rise, New York City subway tunnels flood, polar ice melt and Midwestern farm fields dry up. (AP Photo/Brennan Linsley, File)

Local and international activists march inside a conference center under a giant statue of a spider to demand urgent action to address climate change at the U.N. climate talks in Doha, Qatar, Friday, Dec. 7, 2012. A dispute over money clouded U.N. climate talks Friday, as rich and poor countries sparred over funds meant to help the developing world cover the rising costs of mitigating global warming and adapting to it. (AP Photo/Osama Faisal)

Local and international activists march inside a conference center under a giant statue of a spider to demand urgent action to address climate change at the U.N. climate talks in Doha, Qatar, Friday, Dec. 7, 2012. A dispute over money clouded U.N. climate talks Friday, as rich and poor countries sparred over funds meant to help the developing world cover the rising costs of mitigating global warming and adapting to it. (AP Photo/Osama Faisal)

Local and international activists march inside a conferences center to demand urgent action to address climate change at the U.N. climate talks in Doha, Qatar, Friday, Dec.7, 2012. A dispute over money clouded U.N. climate talks Friday, as rich and poor countries sparred over funds meant to help the developing world cover the rising costs of mitigating global warming and adapting to it. (AP Photo/Osama Faisal)

Local and international activists march inside a conferences center to demand urgent action to address climate change at the U.N. climate talks in Doha, Qatar, Friday, Dec.7, 2012. A dispute over money clouded U.N. climate talks Friday, as rich and poor countries sparred over funds meant to help the developing world cover the rising costs of mitigating global warming and adapting to it. (AP Photo/Osama Faisal)

FILE - This Nov. 26, 2012 file photo shows organizers on stage at the opening ceremony of the 18th United Nations climate change conference in Doha, Qatar. The amount of heat-trapping pollution the world spewed rose again last year by 3 percent. So scientists say it's now unlikely global warming can be limited by more than a couple degrees, which is an international goal. (AP Photo/Osama Faisal, File)

FILE - This Nov. 26, 2012 file photo shows organizers on stage at the opening ceremony of the 18th United Nations climate change conference in Doha, Qatar. The amount of heat-trapping pollution the world spewed rose again last year by 3 percent. So scientists say it's now unlikely global warming can be limited by more than a couple degrees, which is an international goal. (AP Photo/Osama Faisal, File)